Single pillars made of tree trunks called Irmensul ('giant column') representing the 'tree of the universe' were set up on hilltops by some German tribes. A highly venerated Irmensul in what is now Westphalia was cut down by the Christianizing Charlemagne in 772.
With the encouragement of Pope Saint Gregory the Great in the 6th century CE, a common practice among proselytizing Christians was to graft Christian theology onto pre-existing pagan rites and sacred places. In the case of pagan tree cults, this may initially involve the destruction of the sacred grove or the cutting down of a sacred tree. However, it would appear that frequently a church would be built on the same site, thereby co-opting it in the service of Christian conversion. The process effectively Christianized the sacred powers or energies of the original site. Examples of this include the medieval Gothic cathedral of Chartres, which was built on a site which was once sacred to the Celtic Druids (acorns, oak twigs, and tree idols in the scultural decorations on the South Portal of the cathedral may allude to the original Druidic oak grove). And before the Druids, during the Neolithic period, the same site may have been a sacred burial mound.
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